This invention relates generally to a gas analyzer for the control of the combustion process involving oxygen and the combustible elements of hydrocarbon fuels. The usual source of oxygen is air which contains approximately 1 part oxygen and 4 parts nitrogen. If too much air is supplied in the burning process heat must be expended to raise the temperature of the excess oxygen and nitrogen molecules to the stack temperature. Too little air results in a "fuel rich" combustion with the appearance of hydrogen and carbon monoxide in the flue gases, with a consequent waste of fuel. Good combustion control is achieved by the careful control of the concentration of oxygen (O.sub.2), carbon dioxide (CO.sub.2), and carbon monoxide (CO) in the flue gases. In practice the objective is to obtain combustion that is just slightly "fuel lean".
If only one gas is monitored the choice is O.sub.2 or CO. However in most industrial situations where a high premium is placed on the minimum loss of heat up the stack, the concentrations of two of the three gases are determined for the control process. Early measurement systems used extractive analyzers where the gases were analyzed away from the problems of heat and vibration of the boiler and stack. Careful sampling techniques were required to preserve the integrity of the sample as it was transported from the stack to the point of measurement. However the delay of several minutes between the time of the sample extraction in the stack until the data could be supplied for change of control conditions led to the development of the in-situ CO monitor based on the principle of selective absorption of the CO molecule in the infrared spectrum.
An attempt is made in all infrared analyzers to select a spectral interval of frequencies which provides a good match of the source/detector characteristics and the spectral absorption curve of the gas. Ideally the instrument should not respond to any other gas which may be in the sampling region. Unfortunately these conditions are seldom realized and particularly in the case of carbon monoxide which must be measured at low concentrations (about 100 ppm), high concentrations of interfering water vapor and carbon dioxide are always present in the stack. The combustion of hydrogen and oxygen and carbon and oxygen causes water vapor and carbon dioxide to be present. Reasonably low levels of the CO.sub.2 interferent may be achieved with a spectral filter having a very low transmittance in the wings of the filter. Water vapor absorption in the CO band, however, can be as large as the CO absorption at elevated stack temperatures (200 C.). The gas filters (transparent gas filled cells) used in some instruments provide a better match of the gas absorption spectral lines than optical filters but they also suffer from interference by CO.sub.2 and H.sub.2 O.
Insufficient attention to the large variation in the absorptivity of a gas with temperature and to a lesser extent pressure has been given in present day in-situ analyzers. The spectra of any gas which absorbs in the infrared consists of a progressive series of lines at fixed or constant spectral frequencies. The width of the spectral line is a function of the pressure and temperature but more importantly the absorption strength of each line varies with the temperature and the quantized ground-level energy E of each line. The effect of this variation of E with each line is to reduce the line strengths in the center of the band and to increase the line strengths in the wings of the band as the temperature of the gas species increases. The indicated absorption by carbon monoxide as seen through a filter positioned at the center of the CO gas absorption band will significantly decrease with a rising temperature, i.e. the gas has negative temperature coefficient. The CO when seen through a similar filter with a spectral peak in the wings of the gas band exhibits a positive temperature coefficient. This is the reason the shape of the gas concentration curve is often observed to change in instruments where the filter temperature is not precisely controlled. The spectral peak of the filter varies with its temperature causing a change in the gas curve both in magnitude and shape.